asside: remove author and related logic, replace date with lastmod, tweak formatting

layouts/partials/asside.html

@@ -1,9 +1,6 @@
 {{define "aside" }}
     {{ if .Params.description }}<p>{{ .Params.description }}</p>{{ end }}
-    {{ if or (.Params.author) (.Params.date) }}
 	<p>
-            {{ if .Params.author }}By {{ .Params.author }}{{ if .Date }}, {{ end }}{{ end }}
-            {{ if .Date }}{{ .Date.Format "2006-01-02" }}{{ end }}
+	    {{ if .Lastmod }}Last Modified: {{ .Lastmod.Format "2006-01-02" }}{{ end }}
 	</p>
 {{ end }}
-{{ end }}